There are an increasing number of radio applications requiring communication over a wide frequency spectrum, for example, 30-400 megahertz. Radios having circuitry which performs conversion of the received AM signal directly to the audio signal, without first mixing down to IF, are commonly referred to as direct conversion, or zero-IF, receivers.
Using this type of reception, some form of AC coupling or high pass filtering is necessary to remove the inevitable DC offset resulting from the mixing and amplification circuitry in the front end of the receiver. If the signal received is an AM signal and if the translated carrier frequency falls below the corner frequency of the AC coupling, the carrier component of the signal becomes attenuated. Use of conventional AM demodulation algorithms results in a certain degree of distortion in the signal at the demodulated output of the receiver. In many applications, the degree of distortion involved is entirely unacceptable and has required extensive filtering and reconstruction of the received signal.
There is therefore a need for an arrangement and method of demodulating an AM signal in a zero-IF radio receiver which overcomes the afore-mentioned deficiencies.